Large electrical and thermodynamic machines in the power station sector are increasingly being operated nearer to the load limits of design and materials. Sensors are provided in order to detect a local overload, particularly on the rotating parts such as a rotor of such a machine. The sensors must be capable of being read wirelessly from outside. At the same time, there is particular interest in the determination of temperature distributions and strains.
There are different options for the wireless transmission of data measured by the sensors from the moving part of the machine and for the sensors themselves. One such option consists in the use of electrically operated sensors with a radio or infrared interface (IR) for data transmission. One of the disadvantages of this electrical option is that the sensors typically require auxiliary energy which must be provided to the rotating part with great complexity.
To avoid this disadvantage, there is a preference for using passive optical sensors, for example optical conductors with integral fiber Bragg gratings (FBG) as sensors for temperature and strain. These sensors are interrogated, i.e. a sensor signal is generated, by light being coupled into the optical conductor, for example, and the reflection behavior of one of the FBGs observed. A change in the reflection behavior suggests a strain or compression of the FBG due to mechanical loading or temperature change.
Free beam coupling by means of a lens collimator is required to bring the light to be coupled from the stationary evaluation unit to the moving machine part. The adjustment of the lens collimator is very complicated however, as typically a monomode fiber with a core diameter of only 5 to 9 μm is used as the optical conductor on the moving machine part. The spacing of the collimators from one another, i.e. the distance to be covered by the free light beam, is usually between 1 mm and 2 m. At the same time, the collimators can be mounted exactly on the axis of the machine so that light can be transmitted continuously. They can also be mounted outside the axis, wherein, in this case, light can only be transmitted when the collimators move past one another as part of the rotary movement.